Polyoma virus functions important in the initiation and maintenance of cell transformation, and in tumorigenesis, will be studied. Reagents which modify only single-stranded DNAs will be used to mutagenize specific sites in the viral genome. Reactive sites in the "early" region of the genome will be produced by resection of the DNA after nicking with the Eco RI endonuclease. The modified DNAs will be plaque-purified under non-selective conditions, and cloned virus stocks will be screened for alterations in viral functions affecting cell transformation. "Cryptic" viral gene functions which are not required for vegetative growth may be detected.